The invention relates to a remote-control system comprising a main unit and, connected at a distance via a pair of conductors to the main unit, an operating unit which is provided with a series circuit of a number of diodes connected between the conductors and a number of switches which are connected parallel to a group of one or more diodes, the main unit being provided with a supply circuit connected to the conductors and a voltage detection circuit connected to the conductor in a manner such that the voltage detection circuit delivers a detection signal which is dependent on the state of the switches.
A remote-control system of this type is known from the U.S. Pat. No. 4,427,904. In the known system the supply source consists of a voltage source which is connected in series with a resistor to the conductor pair. The resistor serves as a current limiting resistor and to separate the voltage source, with constant output voltage, from the conductor connected to the resistor so that a voltage can appear on the said conductor which is dependent on the state of the switches.
The known system has the drawback that the user of the operating unit receives no information on the correct detection of the position of the switches of the said operating unit and also cannot receive any other messages from the main unit via the conductor pair. This constitutes in particular a disadvantage on using the system in a hospital where a number of operating units are allowed to patients, it being desired that a patient receives a confirmation from his operating unit that the state of the switches has been detected.
The object of the invention is to remove the disadvantage of the known system.